I'm rather unfamiliar with cell phones since I just got my first one earlier this week so I have a question.

I got a Samsung a930 and whenever i try to text certain friends it says "service not available" now from the same place I am able to get ahold of others through text...does this mean they don't have a texting plan on their cell phone. Or is it my cell and my reception? Or could it be something else? I usually have 2-3 bars from where I am at home.